Quadrature amplitude modulation (QAM) is a type of multi-phase modulation that uses two carriers, an in-phase carrier and a quadrature carrier. Each carrier is modulated respectively to e.g. 2, 4 or 8 modulation states or levels. Thus a multi-amplitude modulation offers, e.g. 4, 16 or 64 states corresponding to the initials 4-QAM, 16-QAM and 64-QAM. The grouping of these states is known as a constellation. In order to demodulate a QAM signal it is desirable to recover the carrier signal from the modulated signal.
There are several known methods of carrier recovery in QAM systems. One type is the selective type digital Costas loop. In contrast to a conventional Costas loop, selective type Costas loops process the baseband phase error signal, generate phase compensation control signals, and then selectively feed these signals to a voltage controlled oscillator (VCO). Specifically, a logical control circuit in the loop passes the phase error signal to the VCO only when the concurrent in-phase and quadrature components of the phase error signal both belong to a predefined rectangular region in the IQ plane.
FIG. 4 shows the in-phase and quadrature coordinate space for 16-QAM where the horizontal axis represents the in-phase component and vertical axis represents the quadrature component. The predefined rectangular regions are shown by dashed lines. Problems occur when phase errors in the recovered carrier signal cause the constellation to be rotated such that a received signal falls in a detection region corresponding to a different signal. This occurs in shaded regions 1 and 2 of FIG. 4. When this happens the reference carrier output signal of the VCO is driven to the wrong phase. For example, in a 16-QAM system as shown in FIG. 4, a signal in the middle ring produces a false error signal (self noise) and appears to be a signal from the outer ring. This causes the Costas loop to try to lock the carrier at a phase which results in a tilted 15 constellation. FIG. 5a illustrates the constellation in the correct position. FIG. 5b illustrates a constellation which has become tilted due to the Costas loop locking the carrier on point A.